1. Field of the Invention
This invention relates to liquid recording medium suitable for use in a recording method which performs image recording by ejecting and spattering the liquid recording medium in the form of droplets from an orifice of a nozzle onto a recording member. More particularly, the invention is concerned with a novel liquid recording medium with various properties of remarkable improvement such as stability and responsiveness of the droplets at their ejection from the discharge orifice, stability in storage over a long period of time, affinity for recording members, quality of the image to be recorded, and so forth.
2. Description of the Prior Arts
So-called non-impact recording methods having recently drawn public attention, because uncomfortable noises during the recording operation could be reduced to a negligible order. Among these particularly important is the so-called ink jet recording method which allows high-speed recording on a plain paper without particular image-fixing treatment, and, in this particular field, there have been proposed various approaches including those already commercialized, and others still under development.
Such ink jet recording method is to perform recording by spattering the liquid recording medium, or the so-called "ink", in the form of droplets and adhering the same onto the recording member.
Such ink used for jet recording method is required to have various characteristics depending upon the recording mode as well as characteristics essential to liquid recording medium for printing such as offset printing or writing.
With a view to satisfying such various conditions, there have heretofore been made various proposals. For instance, U.S. Pat. No. 4,106,027 discloses a liquid recording medium having a low viscosity of 5 c.p. or below and an electrical conductivity of 10.sup.-4 ohm.sup.-1 cm.sup.-1 or above, and which is principally composed of an organic solvent. The liquid recording medium is used in a recording device of a type in which the droplets are charged. For the organic solvent, the prior patent uses, as its principal component, alkane or cycloalkane having the carbon content of from 5 to 8, lower alcohols, ethers (diethylether, dioxane, tetrahydrofuran), aromatic hydrocarbons, halogenated hydrocarbons, or esters. The recording agent components used in this system are solvent-soluble anthraquinone dyes, azo dyes, xanthene dyes, phenacine dyes, oxazine dyes and the like.
On the other hand, there have also been known many kinds of liquid medium component system containing water. For example, U.S. Pat. No. 3,687,887 discloses an aqueous system liquid recording medium consisting of an organic resin binder such as styrene-maleic anhydride, etc., ether of polyhydric alcohol (particularly, glycol), and a recording coloring agent (carbon black and dyestuff suspended in water) agent so as to obtain a recorded image having high image density and good image-fixing property on the image forming base such as gelatin, resin film, etc. The recording agent used here is a mixture of carbon black suspended in water and an orthochromatic dye (direct dye or acid dye).
Further U.S. Pat. No. 3,705,043 teaches a liquid recording medium consisting of a recording agent, a wetting agent (polyhydric alcohol, alkyl ether of polyhydric alcohol, or a mixture thereof), and water, with a view to obtaining an infrared ray absorptive image, exhibiting appropriate viscosity value over a long period of time, and not bringing about clogging of the discharge orifice. As the recording agent, there are disclosed infrared absorbers such as water-soluble nigrosine dyes, spirit-soluble nigrosines, modified water-soluble nigrosine dyes, water-dispersed carbon black and mixtures thereof.
U.S. Pat. No. 3,776,742 discloses an effective liquid recording medium having electric conductivity and surface tension of 35 to 70 dyn/cm, and consisting of a water soluble dyestuff, an electrically conductive substance (metal chlorides, etc.), a low molecular weight polyol, and a crystalline organic compound such as urea, etc. This aqueous system liquid recording medium is effective in quick image-fixing on a recording member such as paper which contains therein cellulose. As a water-soluble black dye, there may be mentioned direct dyes of a biphenyl derivative type, acid dyes of a metal complex type, reactive dyes having a pyrimidyl and the like.
However, it is considerably difficult to obtain a liquid recording medium capable of satisfying various requisites simultaneously, and there remain problems to be solved.
That is, this kind of recording method, i.e., ink jet recording method, comprises ejecting a liquid recording medium through a discharge orifice of a minute diameter (usually 10-20 .mu.m) in a recording head and spattering in the form of droplets for recording, and therefore, it is necessary that the method is of high signal responsiveness and high fidelity reproduction.
In addition, there are requested various characteristics, that is, the liquid recording medium can pass through a nozzle at a speed corresponding to the recording speed; after recording, the liquid recording medium can be rapidly fixed to the recording member; after fixed, the recorded images are of high light resistance, high water resistance and high weatherability; the recorded images have a sufficient density; the shelf life is long; there occurs no clogging in orifices and nozzles; and so on.
When an electric or electrostatic method is employed to form or control the liquid droplets, electric or electrostatic property of the liquid recording medium should be appropriately selected. In order to satisfy those conditions, viscosity, surface tension, resistivity, electric capacity, dielectric constant and the like of the liquid recording medium are appropriately adjusted.
The liquid recording medium is fundamentally composed of two components, that is, a recording agent which serves to form recorded images and a carrier liquid which carries the recording agent. However, in practice, a recording agent affects the characteristics of the liquid recording medium to a great extent and therefore, it is very difficult to obtain a liquid recording medium of desired characteristics by using only two components.
Therefore, various additives are added as third components so as to adjust, for example, viscosity, surface tension, resistivity and the like.
However, it is not easy to adjust each of the various characteristics independently by adding the third component. For example, when a viscosity adjusting additive is added to a liquid recording medium which surface tension is adjusted to a desired value, for the purpose of adjusting the viscosity, the other physical properties, in particular, surface tension, are changed sometimes.
Even if desirable characteristics of a liquid recording medium are obtained when it is prepared, it is often very difficult to maintain such desirable characteristics at the time of preparation after long use, long storage or long allowing to stand.
In particular, concentration of non-volatile components such as the recording agent and the like increases as a result of vaporization of carrier liquid components and thereby the physical properties such as surface tension and the like vary to a great extent sometimes. As the result, good recording can not be effected and further, sometimes liquid droplets can not be formed at all because the liquid path in the device is clogged.
In addition, it is desired to select density of recorded images at a wide range, or to select concentration of the recording agent in the liquid recording medium at a wide range without varying physical properties of the liquid recording medium for the purpose of improving reproducibility of half tones, sharpness and the like.
In case of conventional recording agents, the concentration greatly affects surface tension which has a great effect on various important characteristics such as ejection stability, liquid droplet forming property, flowability in the conduit of the device, signal responsiveness and the like. Therefore, it is necessary to control the physical properties at each time when the liquid recording medium is prepared with a different concentration of the recording agent.
Therefore, the recording agent should be such material that does not affect physical properties, in particular, surface tension, of the liquid recording medium upon adding or changing the concentration. In addition, the recording agent should have excellent light resistance, weatherability, fixability to recording members and high sharpness and further a property that it does not precipitate by crystallization or coagulation.